Inverter inrush current

[JVR]

My brother works on earth moving equipment.  His bakkie (ute) has a 800W 230V inverter that needs to drive a 550W drill and a 650W grinder.

The problem is that the inverter can't start the tools at full power, this is ok with the drill, as he can speed it up slowly using its own speed control, but the grinder just trips the inverter.

What I've thought of was to build a "dimmer" for the inverter, so he can start the grinder at a low speed, but this is not an ideal method.  Can a softstarter be fitted? Or is there another way of helping the inverter deal with big inrush currents?

Thanks!
JVR

[Zero999]

The grinder probably uses an induction motor.

You can buy proper soft start for it or running it from a variable frequency drive will also work

[JVR]

VSD would be nice, but its completely out of budget

Being a digital type, what would happen if he fitted a 400V cap over the lines? The way pool motors have them fitted?
Like so?

[Psi]

I wonder what would happen if you put a ~39uF 250VAC capacitor in series with the grinder.
39uF @ 50hz would limit the current to 2.8A (644W)
I have no idea what sort of effect it would have on the grinder motor though.

If the grinder started spinning you could then bypass the cap with a switch or relay+timer when it was up to speed.

hehe, note the word "bypass", not "short out"
39uf @ 230v would be the end of the relay i think .

[mikeselectricstuff]

The grinder probably uses an induction motor.

You can buy proper soft start for it or running it from a variable frequency drive will also work

Only if it's a bench grinder - an angle grinder will be a universal motor like the drill - you could soft-start the latter.

[IanB]

What happens if you plug in the grinder before powering up the inverter?

[NiHaoMike]

Modify the inverter to perform V/Hz overload mitigation. Basically, all it does is current limit the output while keeping the V/Hz relationship constant so that motors will operate properly. I have not seen a general purpose inverter with such a feature, even though it would be very helpful for solving a common problem.

You can also try connecting a hair dryer or other high power resistive load in series.

[JVR]

What happens if you plug in the grinder before powering up the inverter?

Still trips out.   

Thanks for the idea's thus far gents.  He is 500km from me, so I need something that I can courier to him, with the minimum setup.   Anyone have a design for a ghetto softstarter?

[Zero999]

The grinder probably uses an induction motor.

You can buy proper soft start for it or running it from a variable frequency drive will also work

Only if it's a bench grinder - an angle grinder will be a universal motor like the drill - you could soft-start the latter.

If that's the case, phase control would probably work.

I wonder what would happen if you put a ~39uF 250VAC capacitor in series with the grinder.
39uF @ 50hz would limit the current to 2.8A (644W)
I have no idea what sort of effect it would have on the grinder motor though.

That would increase the current unless the capacitance is dominant.

Remember in a series RLC circuit:
Z = sqrt((X_L- X_C)² + R²)

So unless X_C is more than double X_L the current would be higher with the capacitor than without it.

[Psi]

I wonder what would happen if you put a ~39uF 250VAC capacitor in series with the grinder.
39uF @ 50hz would limit the current to 2.8A (644W)
I have no idea what sort of effect it would have on the grinder motor though.

That would increase the current unless the capacitance is dominant.

Remember in a series RLC circuit:
Z = sqrt(X_L- X_C + R)

So unless X_C is more than double X_L the current would be higher with the capacitor than without it.

true, i didn't think of that.

[kiyotewolf]

I've dealt with this myself also, I'm assuming his inverter is either 12 or 24 volts DC input.

I'm betting that it's the same thing as what I was having with mine.
I was trying to use a small hand saw with mine, and it cuts out, only gets it going enough to rotate the blade for a second.

I have not tested this yet, so I don't know how effective it is.

I was going to get some of those car audio capacitors along the power rails, as close to the terminals of the inverter as possible, to handle the instantaneous spike in power consumption that occurs when the load starts.

I was having low voltage cut outs, so I figure, get 1 to 3 capacitors, to keep the constant current and handle increased demand.



~Kiyote!

[Zero999]

I was going to get some of those car audio capacitors along the power rails, as close to the terminals of the inverter as possible, to handle the instantaneous spike in power consumption that occurs when the load starts.

I was having low voltage cut outs, so I figure, get 1 to 3 capacitors, to keep the constant current and handle increased demand.

I doubt that'll work.

Inverters work by boosting 12V to 170VDC or 325VDC (depending on the desired mains voltage) first before converting it to 120V or 230VAC. If the DC-DC converter's peak current rating is exceeded then it will cut-out causing the output voltage to fall.

If the low voltage cut-out is a problem and the battery is fully charged, it means the battery either needs replacing, is underrated or the cable is to long or undersized. Those huge capacitors are a scam.

[PetrosA]

800W probably just plugs into the lighter (power outlet nowadays ). I think he needs a 1500W or 2000W inverter connected directly to the battery to run that type of tool. They cost more, but if he's working that far from civilization, he needs dependability, not cost savings.

[Simon]

I think there are inverters that cost a little more but are capable of starting inductive loads close to their peak output, basically you either derate a cheap inverter (so spend more on a bigger one) or spend the same amount on a decent inverter of a lower power